The invention relates to the field of precise measurement of RF power, and in general to the compensation of radio system RF losses, particularly the compensation of radio system RF losses using closed loop gain compensation.
In radio installations generally, the amount of radio frequency (RF) energy transmitted at the antenna is desirably held consistent from one installation to another. However, many sources of variation in each device result in significant variations. In ground-based single channel communication, the satellite accounts for variations by transmitting command signals to the ground-based unit to increase or decrease power output during transmission. Multichannel communication system installations for use in mass transportation vehicles, such as commercial air transport aircraft, are more complex.
Multichannel communication systems accept data and voice from various sources onboard a vehicle, encode and modulate this information to appropriate RF carrier frequencies, and transmit these carriers over any of multiple transmission channels to the satellite constellation for relay to the ground. Multichannel satellite communication (SatCom) systems also receive RF signals from the satellite constellation, demodulate these signals, perform the necessary decoding of the encoded messages, and output data or voice for use onboard the vehicle by crew members and passengers. Transceivers in such multichannel mobile satellite communication systems include a main system CPU for performing the actual transmit and receive functions, a radio control subsystem that allocates transmission channels to calls, a high power amplifier for boosting the channel power, a common antenna receiving and transmitting signals, and a low noise amplifier amplifying the RF signal received from a satellite. In multichannel mobile communication systems, such as an aircraft installation, many sources of variation in each installation result in significant installation-to-installation variations. For example, typical aeronautical SatCom system installations divide the system functions into multiple separate modules, including a telecommunications module housing the main system CPU and the radio control subsystem, a high power amplifier module, a low noise amplifier module, and the antenna. One important source of variation is inconsistencies in the equipment manufacture. Another important source of variation is the use of different types and lengths of wiring, usually coaxial cable, to interconnect the various physically separated modules, or components, of the communication unit. Although the various functional modules are interconnected with standardized wires or cables for inter-module control and to connect RF signals, installation-to-installation cable type and length variations produce variations in the amount of RF energy at the antenna.
While the desirability of holding the amount of RF energy transmitted at the antenna consistent from one aircraft installation to another is recognized, the necessary use of different types and lengths of cable in different aircraft installations is overcome only by a universal standard cable type and length. Such a standard cable is necessarily the cable required for the most demanding application. Thus, installation-to-installation consistency would require every aircraft to carry the longest, heaviest coupling cables. However, in aircraft installations, the addition of excess cable length and weight is not desirable. Furthermore, a universal type of cable may not satisfy the requirements of all radio installations. Therefore, the variations must be compensated in another way.
The satellite attempts to account for these and other variations in amount of RF energy transmitted at the antenna by transmitting command signals to the communication unit to increase or decrease power output during transmission. In multichannel aircraft installations, the radio control subsystem of the communication unit dynamically controls the output power for each radio transmission channel. The typical communication unit uses closed loop power control algorithms, such as an automatic gain control circuit, for controlling RF power levels at the antenna. The transmitter communication unit receives transmit power level commands from the network satellite, which are intended to control the amount of power radiated by the antenna. The automatic gain control circuit causes the radio control subsystem to increase or decrease power output on each active radio transmission channel in response to command signals transmitted from the satellite. However, the changes in output power applied by the radio control subsystem are not translated consistently into output power at the antenna because the differing amounts of power absorption by the RF cables interconnecting the various modules results in variations in the coupling losses between the radio control subsystem and the antenna. These variations cannot be compensated by the automatic gain control circuit. Such losses may range anywhere from 0 to 20 dB or more, depending upon the installation.
Thus, even with standardized intermodule wiring, each installation results in different amount of cable loss relative to other similarly wired installations. This variation in RF cable loss presents problems with the closed loop power control algorithms of many second generation satellite systems. The installation-to-installation differences in the amount of RF cable loss causes variations in the amount of RF energy at the antenna. Thus, these installation-to-installation variations in cable loss produce variations in the amount of power radiated by the antenna. Such variable losses in a RF transmission system require an accurate RF power measurement for closed-loop power control.
Manual control of the radiated power variations is impractical. For example, attempting to reduce the installation-to-installation variation by tightly controlling the cable types and cable lengths results in a significantly more difficult installation. Manually measuring power levels and manually adjusting the gain of the high power amplifier until a specified power level is measured also results in a significantly more difficult installation.
Furthermore, in installations where standard cabling is provided, adding a cable for a new purpose, such as determining the output power at the antenna relative to the output power at the transmission channel, is not a practical option. Therefore, the detection and communication of system losses must utilize existing cables. One attempt to resolve the coupling losses between the radio control subsystem and the antenna added a DC bias on the return cable from the antenna to the radio control subsystem. However, the DC bias is subject to the same cable losses as the original signal.
Another difficulty presented by the prior art is actual measurement of the power level during a transmit period of a signal that is modulated with digital data. The nature of digital satellite communications is to transmit in short, unpredictable bursts. This short RF burst transmission causes a typical aeronautical SatCom system to operate normally with a transmission method having a duty cycle of less than 100%, and possibly less than 10%. Accurate measurement of the power level during such short and unpredictable transmit periods cannot be achieved using traditional methods.
Conventional measurement systems use generic power measurement methods. Each measurement systems has drawbacks with respect to the manner in which it reports a RMS (Root Mean Square) value for a given signal. The typical method expects a continuous wave (CW) RF transmission, and the measurement systems is tuned to generate accurate RMS values at a given frequency.
FIG. 1 illustrates a typical Phase Shift Keying (PSK) modulation scheme. The In-phase portion is shown as I, and the Quadrature-phase portion is shown as Q. When I and Q are both zero, no output signal is generated. The I and Q portions are modulated in time to represent bits. There is a transition period Xn between each bit. FIG. 1A illustrates the bit periods in time, denoted by labels A, B, C, D and E, and illustrates intervening transition periods Xn in time, denoted by labels X1, X2, X3, X4, X5, X6. FIG. 1B illustrates the bits using phase representation. In FIG. 1B, a hypothetical 5-bit burst is illustrated using bit periods A, B, C, D and E and transition periods X1, X2, X3, X4, X5, X6. The actual power envelope for this hypothetical example is shown in FIG. 1C. Only the average power per bit is necessary for loss compensation; the power during transition periods Xn is not needed.
The output of typical power measurement devices are shown in FIG. 2A. Traditional power measurement methods are inherently inaccurate due to assumptions made during design of the device. For example, determinations whether to use Peak detection methods, and over which time period to average the power input to determine an RMS voltage from the RF wave input. The various curves shown in FIG. 2A illustrate the power reported by traditional power measurement methods. FIG. 2A illustrates the power reported as a function of time for each of a high speed detector, curve A, reporting the instantaneous power level; a peak detector, curve B, and a xe2x80x9ctrue RMSxe2x80x9d detector, curve C, which reports essentially the instantaneous power divided by the square root of 2.
One device disclosed by U.S. Pat. No. 6,046,987, entitled Instrument For Measuring Leakage Power Leaking To Adjacent Channel, issued to Tagawa on Apr. 4, 2000, the complete disclosure of which is incorporated herein by reference, attempts to provide channel power measurement for loss compensation in a complex device that measures leakage power appearing in channels adjacent to a main channel when the main channel receives a RF burst signal by using band pass filters to pass the signal components in the frequency bands of the channels adjacent to the main channel, and then measure the power of each signal component which has passed through one of the band pass filters.
What is needed therefore is a means for accurately determining the output power at the antenna during the short, normally unpredictable RF burst transmission periods experienced by wireless telephony. The desired output for loss compensation is shown in curve D of FIG. 2B. The desired output provides actual power during each bit period averaged over the transmission burst period, and reported as xe2x80x9cheldxe2x80x9d during the non-transmitting periods. The system thereby is provided sufficient time in which to respond to fluctuations in the power during which it can adjust the power output for the burst period.
The present invention overcomes the limitations of the prior art by providing a method and circuit for automatically compensating the installation-to-installation cable loss variation without manual intervention, thereby providing easier and less expensive radio installations. The method and circuit of the invention continually provide precise, accurate RF power measurement. When implemented in a radio communication system, the method and circuit of the present invention provide continuous precise, accurate RF power measurement information allowing continuous precise compensation of the radio system power level variations, resulting in a radio system that is robust to radio system variations, such as variations in gain of the radio system high power amplifier due to manufacturing variations, temperature fluctuations, and other factors affecting cable loss. Furthermore, in a radio frequency communications system using transmission methods having a duty cycle of less than 100%, or even less than 10%, the method and apparatus of the invention provide an accurate measurement of the power during the transmit period that cannot be achieved using traditional methods, by determining when to measure RF power, averaging the measurement across an appropriate time period, and holding the power measurement between measurements. The present invention is used in any radio installation where pulsed RF power measurement is required, such as for closed loop power management.
The method and circuit of the invention precisely and accurately determines the power level difference between a signal source and the antenna transmitting that signal. When implemented in a radio communications system, the method and circuit of the present invention provide means for dynamically adjusting the output power at the antenna of a radio system by determining the power level difference between the signal source and the system antenna, and dynamically adjusting the system gain in response to the detected power level difference.
According to one aspect of the present invention, a method is provided for measuring radio frequency power level in a pulsed RF transmission, the method including determining an appropriate time during a radio frequency transmission period to measure RF power; averaging a value of the RF power level measurement across an appropriate time period; and reporting the average RF power level measurement value as compensation information to the RF signal source. Preferably, the RF power level measurement is used for controlling radio frequency power gains.
According to one aspect of the present invention, the appropriate time to measure RF power level during a transmit period is determined as a function of a priori knowledge of a transmission frame protocol practiced by the radio frequency source, i.e., the transmitter. The a priori knowledge includes knowledge of the modulation scheme and burst timing practiced by the radio frequency transmission system. Determining a time during a transmit period to measure the RF power level also includes determining the peak periods for each of a plurality of bits contained in the RF transmission.
According to another aspect of the present invention, averaging a value of the RF power level measurement includes measuring the RF energy contained in each bit and averaging the measured RF energy. Preferably, the RF energy measurement is accomplished using a Square Root (I*I+Q*Q) formula.
According to another aspect of the present invention, reporting said average RF power level measurement value includes reporting the average power measurement value as constant between individual measurements. Compensation of the RF power level depends upon reporting the average power measurement value to the source generating said RF power.
According to another aspect of the present invention, in a RF transmission system, the RF power level measurement is preferably accomplished at a point in the RF transmission system where any radio frequency power losses between said measurement point and the radio system antenna are essentially constant and known. Preferably, the power level information is reported to the RF source via preexisting receiver cabling within the radio system. Preferably, the reporting structure includes generating a power level information signal indicative of the RF power level measurement and combining the power level information signal with an externally generated radio frequency signal received by the host RF transmission system. The power level information signal is preferably a frequency signal in a frequency range different from the frequency of the radio frequency transmission originated by the host RF transmission system.
According to various other aspects of the invention, the power level information signal is a sinusoidal signal proportional to said radio frequency power level of the radio frequency transmission. Alternatively, the power level information signal is a frequency modulated digital signal having digital information contained in the modulation, the digital information is representative of the average RF power level of the pulse transmission.
According to yet other aspects of the invention, the method of the invention is realized in a electronic circuit adapted for use with a source of a pulsed RF transmission. For example, the circuit of the invention is used to compensate RF power level a RF communications system generating pulsed RF transmissions. In such applications, the circuit of the invention is preferably placed at a position in the transmission line where further RF power losses between the circuit and the system antenna are small relative to other losses in the system and essentially known, such that the further losses appear essentially as a constant off-set in the transmission power.
According to still other aspects of the invention, the method and circuit of the invention are capable of supporting multiple communication channels. In particular, portions of the circuit of the invention are multiplied to support each additional channel, up to a maximum number of channels. Accordingly, the number of down converters and analog-to-digital converters in the circuit is increased by one each for each additional channel operated by the invention, thereby providing power compensation on a channel-by-channel basis.